I have found that it's always better to book directly with hotels. The price is the same or better, but more importantly, it's waaaay easier to make changes or cancellations without the middleman.
And rentalcars.com is a flat-out scam. I had to dispute CC charges with them when I showed up on scene, there were no cars, and rentalcars wouldn't refund it. Always book with the rental company directly.
Definitely not my experience -- I find the price is usually better with aggregators.
The hotels sell rooms full-price on their site, and only release cheaper prices to budget aggregators. Often times the cheapest hotel deals are only available as part of a flight+hotel package where the cheaper price never gets revealed at all, because they never reveal which actual proportion of your package goes to the airline and to the hotel (hint: it's usually the hotel that gives the discount, even if the package is advertising a "$0 flight").
You're right about it being easier to change/cancel, for sure. But it's not also cheaper -- it's the opposite. You're generally paying more for that.
This is why when you travel for business and plans change and someone else is paying, book direct. Whereas when you're traveling for fun but on a budget with fixed dates you know won't change, use a budget aggregator.
And rentalcars.com is a flat-out scam. I had to dispute CC charges with them when I showed up on scene, there were no cars, and rentalcars wouldn't refund it. Always book with the rental company directly.